Calcium containing lead alloy anodes for electrowinning

ABSTRACT

An improvement in processes for electrowinning copper from aqueous, sulfuric acid solutions comprises utilizing anodes of an alloy of lead and calcium containing from about 0.025 to about 0.1 percent calcium by weight. Anodes of the lead-calcium alloy have exceptionally superior resistance to corrosion in comparison to conventional lead or lead-antimony alloy anodes when used in electrowinning copper from sulfuric acid solutions.

United States Patent Hood, III. et al.

[4 1 Jan.7,1975

CALCIUM CONTAINING LEAD ALLOY ANODES FOR ELECTROWINNING Inventors:Andrew G. Hood, III.; David L.

Adamson; Terrell N. Andersen; Kenneth J. Richards, all of Salt LakeCity, Utah Assignee: Kennecott Copper Corporation,

New York, NY.

Filed: Feb. 27, 1974 App]. No.: 446,217

US. Cl 204/108, 204/293, 75/101 BE,

75/117, 75/167 Int. Cl C22d 1/16, BOlk 3/06 Field of Search 204/293,108; 75/167 References Cited UNITED STATES PATENTS 11/1941 Shoemaker75/167 11/1966 Jensen 75/167 2/1974 Manko et a1 204/293 PrimaryExaminerR. L. Andrews Attorney, Agent, or FirmMallinckrodt &Mallinckrodt [57] ABSTRACT 4 Claims, No Drawings CALCIUM CONTAINING LEADALLOY ANODES FOR ELECTROWINNING BACKGROUND OF THE INVENTION 1. Field Theinvention relates to processes for electrowinning copper from aqueoussulfuric acid solutions containing dissolved copper values.

2. State of the Art Electrowinning processes are widely used to recovermetallic copper from aqueous electrolyte solutions containing dissolvedcopper valves. Conventional electrowinning processes for copper utilizeanodes made of lead or an alloy of lead and antimony. When an aqueouselectrolyte solution contains a corrosive material, such as sulfuricacid, the lead or lead-antimony alloy anodes are subject to corrosion.Antimonial lead anodes have a generally satisfactory service life whenthe sulfuric acid concentration in the electrolyte solution is small,i.e., below about 80 grams per liter of sulfuric acid. However, withelectrolytes having higher sulfuric acid concentrations, the corrosionof conventional antimonial lead anodes is greatly accelerated, whichcreates a serious problem not only from the standpoint of high anodereplacement costs but also with respect to lead contaminants which aretransmitted to the copper deposited at the cathodes.

The alloying of lead with calcium to obtain a hard lead alloy isdisclosed in U.S. Pat. Nos. 1,158,681; 1,890,014; 2,042,840; 2,170,650;2,198,932; and 3,287,165. The hard lead-calcium alloys are taught asbeing useful in manufacturing electrical cable sheaths, ammunition, andplates in lead storage batteries. In the latter application, thelead-calcium alloy plates were found to be less susceptible toself-discharge which is inherent with conventional lead-antimony alloyplates due to transfer of antimony between the plates of the battery.

SUMMARY OF THE INVENTION The present invention constitutes animprovement in the known process of electrowinning copper from anaqueous sulfuric acid electrolyte solution containing copper ions. Theimprovement comprises utilizing anodes consisting of a lead alloycontaining from about 0.025 to about 0.10 percent calcium by weight. Theterm lead is meant to include pure lead as well as commercial lead andchemical lead, which usually contains small quantities of copper,silver, nickel, zinc and bismuth. This term is also meant to includelead alloys in which the alloying elements do not eliminate thecorrosion resistance provided by the calcium addition.

It has been found that incorporation of small amount of calcium in thelead forms an alloy, which, when used as an anode in electrowinningcopper from aqueous sulfuric acid solutions, is remarkably stable andexhibits exceptionally superior resistance to corrosion in comparison toconventional lead or lead-antimony alloy anodes. Further, the use of thelead-calcium alloy anodes results in significant reduction of leadcontaminants in the cathode-deposited copper.

DETAILED DESCRIPTION OF THE PREFERRED PROCEDURE The best mode presentlycontemplated of carrying out the improved process of the inventioncomprises immersing at least one anode and at least one cathode in anaqueous, sulfuric acid electrolyte containing dissolved copper values,and impressing an electrical potential across the anodes and cathodes todeposit copper on the cathodes. The anodes consist of an alloy of leadcontaining from about 0.025 to about 0.10 percent calcium by weight. Thecalcium-lead alloy which is used in the anodes of this invention can bemade in any conventional manner, including the procedure described indetail in U.S. Pat. Nos. 1,890,014 and 2,042,840.

The invention is applicable to all conventional processes ofelectrowinning copper from an aqueous, sulfuric acid electrolyte.Generally, the electrolyte solution will contain from about 10 to about300 grams of I-I SO per liter of solution, and from about 0.5 to about60 grams of dissolved copper per liter of solution. The electrolyte canbe passed through conventional electrowinning cells which contain alarge number of alternately disposed cathode and anode sheets whereinthe anode sheets are made from a lead-calcium alloy according to thisinvention. The cathodes can be any of those conventionally used in theelectrowinning of copper. For example, the cathodes can be made ofstainless steel, titanium, or electrolytically deposited copper. Thecurrent density can vary over a generally wide range, such as from about5 to about amps per square foot, with a voltage drop per electrode pairof approximately 2 volts.

The improved process of the present invention is advantageously used inelectrowinning copper from dilute, sulfuric acid leach solutions whichhave been used in leaching copper values from copper-bearing material,such as low grade ores and mine waste dumps. The improved process isparticularly advantageous in electrowinning copper from somewhat moreconcentrated sulfuric acid solutions, as those obtained in multi-step,

leach-solvent extraction processes.

In one such multi-step process, copper scrap and/or cement copper isinitially leached with an aqueous ammonical leaching solution underoxidizing conditions to oxidize the copper values to cupric oxide. Thecupric oxide then reacts with ammonium hydroxide in the aqueousammonical leaching solution to form a soluble copper ammonia complex.Thereafter, the copperpregnant ammonical leach solution is passed to anion exchange zone, wherein it is contacted with an organic, ion exchangeliquid which generally contains a liquid organic ion exchange agentdissolved in a suitable, water immiscible, organic solvent, such askerosene. A suitable liquid organic ion exchange agent is a substituted2-hydroxy benzophenoxime, such as taught in U.S. Pat. No. 3,428,449, theentire disclosure of which incorporated herein by reference. Thedissolved copper ions are extracted from the aqueous ammonical leachsolution by the organic ion exchange agent,'and the organic and aqueousphases are then allowed to separate. The copper-pregnant organic ionexchange agent is thereafter contacted with an aqueous, sulfuric acidsolution to strip the majority of the copper therefrom. The aqueous,sulfuric acid solution obtained from the stripping step generallycontains from to 300 or more grams per liter of sulfuric acid and fromabout 20 to about 50 grams per liter of dissolved copper values. Such anion exchange system can also be utilized to transfer copper ions from adilute acid leach solution, (such as the dilute sulfuric acid solutionmentioned hereinabove) to the more concentrated sulfuric acid solutionin a manner similar to that described for transferring copper from anammonical leach solution to a sulfuric acid solution.

The lead-calcium alloy anodes of this invention have been found to beexceptionally resistant to corrosion even when the electrolyte is onewhich contains a substantial concentration of sulfuric acid, such as,for example, the electrolyte from the solvent extraction systemsdiscussed above. In addition, the copper produced by the process of thisinvention contains a minimum amount of lead impurities, usually lessthan 4 parts per million.

The invention is further described in connection with the followingexample which is intended to illustrate the invention but not to limitthe scope thereof.

EXAMPLE A calcium-lead alloy anode was made containing 0.10 weightpercent calcium with the balance being essentially lead. Twoantimony-lead alloy anodes were also made. One containing 5 weightpercent antimony with the balance being lead, and the second containingweight percent antimony and 0.4 weight percent arsenic with the balancebeing essentially lead. Each of these anodes was approximately 170 cm inarea. Each anode was placed into an electrolytic cell so as to bepositioned between two cathodes of equal dimensions, with a cathode toanode spacing of one inch. The cathodes were made of copper or titaniummetal.

An aqueous sulfuric acid electrolyte containing 180 grams per liter ofsulfuric acid, 40 grams per liter of dissolved copper, and 2 grams perliter of dissolved iron was introduced into each of the electrolyticcells to submerge the anode and cathodes of each cell in theelectrolyte. An electric potential was applied between the anode andcathodes of each cell so that each anode operated under a currentdensity of 16 amps per square foot, with a voltage drop of about 2 voltsbetween the anode and cathodes of each cell.

The corrosion rate of each anode was determined by periodicallymeasuring the weight loss thereof. Each anode exhibited an initialinductance time, that is to say an initial period of time after theanode had been put in service, wherein the weight loss was nil. Duringthe induction time, a film of PbO formed on the anodes. After theinduction period, the corrosion rate, or rate of weight loss wasconstant. The corrosion rates of the anodes for the period after theinitial induction time is shown in the following table. The inductiontime for each anode and the length of the test of each anode is alsoshown in the table.

As can be seen from the above data, the anodes made of the lead-calciumalloys were found to corrode at a rate ofonly 2-3 percent of that of theconventional anodes made of antimony-lead alloys. Thus, the life of thelead-calcium anodes would be some 20 to 33 times as long as that of theconventional antimony-lead anodes.

The process taking place in the three cells that utilized the threelead-calcium anodes produced copper in which the lead impurity did notexceed 4 parts per million.

Whereas this invention is described herein with respect to certainpreferred procedures thereof, it is to be understood that manyvariations are possible without departing from the inventive conceptsparticularly pointed out in the following claims.

What we claim is:

1. A process for electrowinning copper from an aqueous sulfuric acidelectrolyte containing dissolved copper values comprising immersing atleast one anode and at least one cathode in said electrolyte andimpressing an electrical potential across said anode and cathode todeposit copper on said cathode, the anode being a lead alloy containingfrom about 0.025 to about 0.10 percent calcium by weight.

2. A process in accordance with claim 1, wherein the anode consists offrom about 0.025 to 0.1 percent calcium by weight, with the remainderbeing lead.

3. A process in accordance with claim 1, wherein the aqueous sulfuricacid electrolyte contains from about 10 to about 300 grams per liter ofsulfuric acid.

4. A process in accordance with claim 3, wherein the electrolytecontains at least 10 grams per liter of sulfuric acid.

1. A PROCESS FOR ELECTROWINNING COPPER FROM AN AQUEOUS SULFURIC ACIDELECTROLYTE CONTAINING DISSOLVED COPPER VALUES COMPRISING IMMERSING ATLEAST ON ANODE AND AT LEAST ONE CATHODE IN SAID ELECTROLYTE ANDIMPRESSING AN ELECTRICAL POTENTIAL ACROSS SAID ANODE AND CATHODE TODEPOSIT COPPER ON SAID CATHODE, THE ANODE BEING A LEAD ALLOY CONTAININGFROM ABOUT 0.025 TO ABOUT 0.10 PERCENT CALCIUM BY WEIGHT.
 2. A processin accordance with claim 1, wherein the anode consists of from about0.025 to 0.1 percent calcium by weight, with the remainder being lead.3. A process in accordance with claim 1, wherein the aqueous sulfuricacid electrolyte contains from about 10 to about 300 grams per liter ofsulfuric acid.
 4. A process in accordance with claim 3, wherein theelectrolyte contains at least 10 grams per liter of sulfuric acid.